Fluid mounts of the "hydraulic damper" type have long been used in vehicular and other applications to dampen shocks and/or vibrations. A typical hydraulic damper has interconnected variable volume chambers between which hydraulic fluid passes during excitation of the mount. Resistance of the fluid to flow between the chambers opposes and damps vibratory and similar forces imposed upon the mount. The viscous damping forces generated by the mount are proportional to, among other things, the viscosity of the hydraulic fluid and the extent to which its flow between the chambers is "throttled" or otherwise impeded by the orifice or conduit through which the fluid passes. The use of hydraulic fluids of relatively high viscosity is therefore acceptable and desirable in many viscous fluid dampers.
A newer type of fluid mount, which has received increasing acceptance within recent years, utilizes fluid inertia forces to achieve and/or to enhance the desired attenuation of vibratory forces. A plot of the dynamic stiffness against the excitation frequency of mounts of the fluid inertia type typically includes a notch-like region, at which the dynamic stiffness of the mount is greatly reduced and may be considerably less than its static stiffness, followed by a "peak" of large dynamic stiffness. A mount may be so designed as to cause the foregoing abrupt variations in its dynamic stiffness to occur at a particular excitation frequency where a specific vibration problem exists. For example, objectional "drone" noise occurring within some automobiles as a result of transmission to their frames of engine firing vibrations generated at a particular engine speed, may be substantially eliminated by the use of an inertia type engine mount that is specifically designed so as to possess its minimum-stiffness "notch" at the frequency of the aforesaid vibrations.
While static mount tuning is satisfactory for the attenuation of troublesome vibrations occurring at only one particular frequency, problem vibrations such as those producing vehicle "drone" noise may occur at a number of significantly differing engine speeds and/or mount excitation frequencies. In such a situation it is highly desirable for a mount to be dynamically tunable so as to permit selective variation during mount operation of the frequencies at which the mount has very low dynamic stiffness. Since the frequency at which stiffness reduction occurs is a function of, among other things, the size of the fluid flow path between the variable volume chambers of a mount, one theoretically possible way of dynamically tuning the mount is by varying the flow path cross-sectional area. In a mount containing a plurality of flow passageways between the chambers, this result should be realizable by selective opening and closing of valve means associated with one or more of the passageways. However, the expense, size and/or relative slowness of operation of conventional mechanically or electromechanically actuated valves makes their use less than satisfactory for the foregoing purpose.
A possible alternative to the use of conventional valves and conventional hydraulic fluids, such as glycol and/or water, is the use of "valves" that generate high voltage electrical fields and of an electrorheological fluid whose apparent viscosity greatly increases in the presence of such electrical fields. Of the two types of fluid mounts, those of the viscous damping type are more naturally suited for the use of electrorheological fluids. The desired generation of viscous damping forces by such mounts tends to be enhanced by the use of such fluids since their viscosity is relatively high even in the absence of an applied electrical field. The viscous damping forces generated by the mount are also enhanced by the "throttling" of the fluid by the relatively closely spaced valve electrodes between which the fluid passes and an electrical field is generated. In an inertia type fluid mount, on the other hand, the aforesaid flow impeding effects oppose generation of the desired fluid inertia forces and the resulting abrupt changes in mount stiffness at certain frequencies. The situation is further aggravated by the fact that the flow resistance produced by the field generating electrodes between which the fluid passes is inversely proportional to the cube of the spacing or "gap" distance between such electrodes. This is significant since such spacing, along with other factors such as the magnitude of the applied voltage, determines the applied stress that the field-actuated fluid can withstand without undergoing shear. The electrode valve in an inertia type mount normally would be required to produce a higher yield point stress in the electrorheological fluid than would the valve in a viscous damper, since in the inertia type mount flow through the valve is to be entirely stopped, whereas in the viscous damper total cessation of the flow would rarely if ever be necessary or desirable.